Cyclic black shale and nannofossil limestone units of the Lower Cretaceous Maiolica Formation of the southern Alps have been studied. From sedimentological data, the black shale units appear to have been formed in an euxinic basin that experienced periodic overturn and basinal aeration. The carbon-13 values of the calcareous nannofossils serve as indicators of paleocirculation. In the Barremian, the palynological spectra of the black shale units reveal a drastic alteration from a warm, dry to a cooler, humid climate. As a result, thermohaline circulation in the western Tethys Ocean was replaced by density stratification, and stagnation of the deeper basins followed.